This week we had a period of weird behavior for the monitoring system for (mostly) German IPv6 servers.
After much back and forth on the mailing list and numerous debugging sessions we got this information from a network engineer at Hurricane Electric:
A bug was recently discovered in Force10 switches that cause unicast IPv6 NTP traffic to be erroneously broadcast to all ports. Due to this, there are currently access lists in place preventing some IPv6 NTP traffic from traversing the DECIX exchange, as it was causing a storm that generated nearly 1 terabit per second of traffic. This should be resolved in the near future.
The number of IPv6 servers active in the pool appears to be about back to normal.
Also this is the answer to "why don't we have IPv6 servers by default on all the pool zones" yet. As you might know only "2.pool.ntp.org" (and 2.debian.pool.ntp.org, etc) returns AAAA records currently.